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Design 18 MoldMaking Technology —— OCTOBER 2017 (even when set correctly) can lock stress into the polymer chain. For example, a mold temperature of 180 o F and a melt temperature near 550 o F yields a certain amount of stress. However, if the molder set the temperature controller to run at 80 o F to mold parts faster, the thermal shock will be higher and the locked-in stress will be greater. A consistent flow rate through the water channels is criti- cal for an even part temperature at ejection. If the tempera- ture range across the part is wide enough, significant shrink- age will occur after molding. This can vary drastically across material and geometry. Generally, when the temperatures vary more than 20 o F across the part, it is time to address the cooling line design. Set the cooling timer on the press to ensure that the part is rigid enough for ejection. There is no general guideline for the timer, which is dependent upon cav- ity geometry, part thickness, polymer type, mold steel type, cooling channel placement, cooling medium or temperature used and process parameters (melt temperature, flow rate, pack/hold time and pack/hold pressure). In some cases, when the timer is set too high, the part will cool dramatically, mak- ing ejection difficult. White stress marks around ejector pins are an indication that the part is not releasing from the mold surface well. If a part has struggled with post-mold warp, passing opti- cal requirements on a lens, chrome adhesion, environmental protocols or drop testing, then it could have too much stress locked into the molded component. Determining whether that stress is related to part geometry, material, mold or pro- cess is crucial to solving this common quality issue. It takes collaboration to minimize that stress on the molded part as well as the people involved. CONTRIBUTOR Jeremy Williams is a consultant/trainer for RJG's TZERO consulting service. He has more than 17 years of experience in the plastics industry serving the medical, automotive, furniture and appliance industries. He earned his Master Molder II certificate in 2011, became a Train the Trainer in 2012 and started at RJG in 2015. FOR MORE INFORMATION RJG Inc. / 616-307-4953 jeremy.williams@rjginc.com / rjginc.com moldmakingtechnology.com/blog Join the Discussion! E N G I N E E R B U I L D M A I N T A I N Conversation. Communication. Community. BL OG • Keep up on the issues affecting mold manufacturing. • Interact with industry leaders and newcomers alike. • Share insights, observations, challenges and solutions with your peers. Participate in the MoldMaking Technology Blog The MoldMaking Technology Blog is designed for you, allowing you to participate in a dialogue among professionals in the mold manufacturing industry.